U.S. patent application number 12/457262 was filed with the patent office on 2009-12-17 for high temperature speed sensor.
This patent application is currently assigned to WESTON AEROSPACE LIMITED. Invention is credited to Nigel Philip Turner.
Application Number | 20090309577 12/457262 |
Document ID | / |
Family ID | 39638324 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090309577 |
Kind Code |
A1 |
Turner; Nigel Philip |
December 17, 2009 |
High temperature speed sensor
Abstract
A gas turbine shaft speed sensor including a sensing coil
comprised of a central conducting wire, the sensor and conducting
wire is surrounded by a layer of mineral insulator and the mineral
insulator is surrounded by a metallic, non magnetic, sheath. A
sensing coil formed with this construction allows the high
operating temperatures and is robust.
Inventors: |
Turner; Nigel Philip;
(Hampshire, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
WESTON AEROSPACE LIMITED
Farnborough
GB
|
Family ID: |
39638324 |
Appl. No.: |
12/457262 |
Filed: |
June 4, 2009 |
Current U.S.
Class: |
324/174 |
Current CPC
Class: |
G01P 3/488 20130101;
G01P 3/49 20130101; F01D 17/06 20130101; F02C 9/28 20130101 |
Class at
Publication: |
324/174 |
International
Class: |
G01P 3/48 20060101
G01P003/48 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2008 |
GB |
0810408.5 |
Claims
1. A gas turbine shaft speed sensor including a sensing coil formed
from mineral insulated cable, the cable comprising: a conductive
wire; a layer of mineral insulation surrounding the conductive
wire; and a metallic sheath surrounding the layer of mineral
insulation.
2. A gas turbine shaft speed sensor according to claim 1, wherein
the mineral insulation includes at least one of magnesium oxide,
aluminium oxide and silica.
3. A gas turbine shaft speed sensor according to claim 1, wherein
the metallic sheath is formed from a non-magnetic metal.
4. A gas turbine shaft speed sensor according to claim 3, wherein
the metallic sheath is formed from stainless steel or a nickel
alloy.
5. A gas turbine shaft speed sensor according to claim 1, wherein
the cable has a diameter of less than 1 mm.
6. A gas turbine shaft speed sensor according to claim 1, wherein
the metallic sheath has a thickness of between 10% and 20% of the
diameter of the cable.
7. A gas turbine shaft speed sensor according to claim 1, wherein
the sensor is a variable reluctance proximity or speed sensor, and
wherein the voltage induced in the coil as a result of changes in
magnetic flux experienced by the coil caused by the presence of an
object in proximity to the coil, is detected by a voltage measuring
means.
8. A gas turbine shaft speed sensor according to claim 1, wherein
the sensor is an eddy current sensor.
9. A gas turbine shaft speed sensor according to claim 8, wherein
the current sensor is an active eddy sensor.
10. A gas turbine shaft speed according to claim 8, wherein the
current sensor is a passive eddy sensor.
Description
[0001] The present invention relates to a gas turbine shaft speed
sensor.
[0002] The use of magnetic sensors in cooperation with, for
example, one or more projections on a shaft to give an output from
which shaft rotational speed or torque can be determined is well
known. In such sensors, a voltage induced in a coil by changes in
the magnetic flux pattern experienced by the coil, caused by
movement of a body of magnetic material in proximity to the coil,
is detected and/or measured.
[0003] This type of sensor has been used in gas turbine engines in
order to sense the speed of the turbine by detecting the teeth of a
phonic wheel passing the sensor. The speed of a rotating gas
turbine shaft is typically monitored by monitoring the movement of
a magnetic toothed phonic or tone wheel, which rotates with the gas
turbine shaft. A magnetic speed sensor monitors the changes in a
magnetic field as a tooth passes it. The passage of each tooth
generates a probe signal pulse and the probe signal train is used
to calculate the rotational speed of the toothed wheel by measuring
the time between successive pulses, or counting a number of pulses
in a fixed time. The rotational speed of the gas turbine shaft is
then derived from the speed of the phonic or tone wheel. The
interior of a gas turbine engine can be a high temperature
environment, and accordingly it is desirable that the sensing coils
used are robust and continue to work at high temperature.
[0004] Proximity and speed sensing coils for gas turbine engines
have typically been constructed from enamel insulated wire. This
limits the working temperature of the coil to around 260.degree. C.
Previous attempts to increase sensing coil working temperature,
such as the use of woven fibreglass, or ceramic fibres have proved
bulky and not robust. Unsheathed ceramic coating on the coil has
been tried, but that has proven delicate and difficult to work
with. Anodised aluminium wire can offer a small increase in working
temperature, to approximately 350.degree. C., but aluminium wire is
not robust and is difficult to join.
[0005] The present invention provides a sensor as defined in the
appended claims, to which reference should now be made. The present
invention provides a sensor including a sensing coil that allows
working temperatures up to around 1000.degree. C., and that is
robust. Preferred features of the invention are defined in the
dependent claims.
[0006] Embodiments of the invention will now be described in
detail, with reference to the accompanying drawings, in which:
[0007] FIG. 1 is a cross-section through a mineral insulated cable
forming a coil for use in a sensor in accordance with the present
invention;
[0008] FIG. 2 illustrates a variable reluctance sensor using a
mineral insulated sensing coil in accordance with the present
invention;
[0009] FIG. 3 illustrates a passive eddy current sensor using a
mineral insulated sensing coil in accordance with the present
invention; and
[0010] FIG. 4 illustrates an active eddy current sensor using a
mineral insulated sensing coil in accordance with the present
invention.
[0011] FIG. 1 shows in cross-section a mineral insulated cable. The
cable comprises a central conductor 10, which is typically formed
of copper, but it may be formed of any other suitable conductive
material. Surrounding the central conductor is a layer of mineral
insulator 12. The mineral insulator is typically formed of
magnesium oxide (MgO), Silica or Aluminium oxide (Al.sub.2O.sub.3).
However, other mineral insulator materials may be used. Surrounding
the mineral insulator layer is a metallic sheath 14.
[0012] Mineral insulated cable of this type is well known and has
been used in coils in industries such as the nuclear industry, for
measuring the shape and position of plasma boundaries (see for
example P2C-D-91, 23.sup.rd Symposium on Fusion Technology, 20-24
Sep. 2004, Fondazione GN, Venice, Italy) and in the metallurgy
industry for measuring molten metal levels (see, for example, GB
1585496).
[0013] Mineral insulated cable of the type shown in FIG. 1 can now
be manufactured with a diameter less than 1 mm, and even as small
as 0.25 mm in diameter. These dimensions make it practical for use
in sensing coils in gas turbine engines and in automotive
applications. Mineral insulated cable of this type forms a robust
coil that allows working temperatures limited only by the materials
within the mineral insulated cable. Typically, this allows working
temperatures up to around 1000.degree. C. In the case of variable
reluctance sensors, as illustrated in FIG. 2, the upper limit of
working temperature is, in fact, limited by the Curie temperature
of the magnet used in the sensor, which is typically around
700.degree. C., rather than by the sensing coil. However, a mineral
insulated cable exciter coil could replace the magnet to further
extend the temperature range if required.
[0014] For use in a sensing coil, the metallic sheath is made from
a non-magnetic material, in order to avoid any interference with
the operation of the sensor. The metallic sheath is typically
formed of stainless steel, or a Nickel alloy such as Inconel 600,
but other metals or alloys may be used.
[0015] Mineral insulated cable can be made by placing copper rods
inside a cylindrical metallic sheath and filling the space between
with dry MgO and/or other insulator powder. The complete assembly
is then pressed between rollers to reduce its diameter.
[0016] Apart from providing an increase in the working temperature
range, another benefit of using mineral insulated coils in the
sensor is that, due to the robustness of the metallic outer sheath,
no additional insulation is required on the parts of the apparatus
which the coil is formed around and is in contact with. Typically,
in a variable reluctance sensor as illustrated in FIG. 2, the pole
piece and end face of the sensor housing needs additional
insulation when used in a gas turbine engine on an aircraft. Even
with previous high temperature designs using glass fibre, ceramic
coated wire, additional insulation is required on the pole piece
and the surrounding metalwork, as the normal insulation is not very
strong and would not withstand a high voltage generated during a
lightening strike. This additional insulation, in the form of glass
fibre, ceramic or mica, is typically bulky, not very robust, and
prone to breakdown. By using a mineral insulated cable of the type
shown in FIG. 1 this additional insulation is no longer
required.
[0017] FIG. 2 shows an example of a variable reluctance sensor for
sensing the rotational speed of a shaft, using a mineral insulated
sensing coil 20 in accordance with the present invention. The
mineral insulated cable forming the coil can have a diameter from
around 0.25 mm to several mm, but it is preferably less than 1 mm.
The thickness of the sheath layer is typically between 10% and 20%
of the diameter of the cable. The mineral insulator layer also has
a thickness of between 10% and 20% of the cable diameter. The
sensor comprises the coil 20 wound around a pole piece 21. The pole
piece is magnetised by a magnet 22. The voltage across the coil is
monitored. A voltage monitoring means is attached to the coil by
leadout wires 23. A phonic wheel, which consists of a toothed
wheel, where the teeth are formed of a magnetic material, is
mounted on the shaft close to the sensing coil. The magnetic flux
in the pole piece 21 (and hence the voltage induced in the coil
20), depends upon the strength of the magnet 22 and upon the
magnetic reluctance of the circuit consisting of the magnet, the
pole piece, the coil, the air gap, the phonic wheel, and the air
path returning the magnetic field from the phonic wheel to the
magnet. As the teeth of the phonic wheel pass the end of the pole
piece the reluctance of the magnet circuit changes, resulting in a
different voltage induced in the sensing coil 20. From the voltage
signal measured by the voltage measuring means 24, the rotational
speed of the phonic wheel, and hence the shaft, can be determined.
A variable reluctance sensor of this type is described in more
detail in EP 1355131.
[0018] The use of a mineral insulated coil in the apparatus shown
in FIG. 2 allows for higher operating temperatures and increased
reliability compared to prior sensors of the same type.
[0019] One of the potential issues with the use of mineral
insulated cable coil, as described with reference to FIGS. 1 and 2,
is whether the sheath material forms a secondary coil, effectively
a shorted turn, which suppresses the output from the primary coil.
The inventor has performed tests comparing the output from mineral
insulated coils and from enamelled copper wire coils. The inventor
found that the sheath does not cause significant problems, as the
sheath material has a relatively high resistivity and a relatively
high contact resistance between turns. Contact resistance depends
on a number of factors, including surface roughness, surface
oxidation and the resistivity of the material. Accordingly there
are steps, such as surface roughening, that can be taken to
increase contact resistance and thereby reduce the impact of the
sheath on the output from the primary coil if required.
[0020] FIGS. 3 and 4 show two further example applications of a
mineral insulated sensing coil. The examples are eddy current
sensors used for measuring jet engine blade passing frequency
and/or blade tip clearance. This is another example of an
application where a coil having a high operating temperature is
required, as the engine casing in a jet engine is often well in
excess of the 250.degree. C. limitation of enamelled wire.
[0021] FIG. 3 shows a passive eddy current sensor using a mineral
insulated sensing coil 30. The passing blades 34 interrupt the
field of the magnet 32 and eddy currents are produced in the
blades. The resulting change in magnetic flux is picked up by the
mineral insulated sensing coil 30. The voltage output from the
sensing coil can then be analysed to determine blade passing
frequency and/or blade tip clearance.
[0022] FIG. 4 shows an active eddy current sensor in which the
mineral insulated sensing coil 40 produces its own magnetic flux.
The passing blades 42 interrupt the magnetic field created by the
excited coil 40 and eddy currents are produced in the blades. The
resulting changes in magnetic flux induce different voltages within
the coil 40. The induced voltage can then be analysed to determine
the passing frequency of the turbine blade and/or blade tip
clearance.
* * * * *